A machine of this type is characterized, in operation, by a continuous advance of the main chassis on which is superposed a discontinuous advance of the mobile chassis supporting the working members. This latter must, in fact, particularly in the case of a track tamping machine, start up, brake and stop at each work cycle, over a relatively short path of displacement determined for example by a gap of 50 to 60 cm between the ties of the track.
Railway track tamping machines designed to this end are already known, in which the mobile chassis bearing the tamping and lifting/shifting members is composed of a chassis resting at the rear on an axle circulating on the rails between the rolling members of the main chassis of the tamping machine. This chassis is provided in its front part with two drawbars sliding with respect to the main chassis in appropriate guiding devices. The mobile chassis is accelerated and braked at each work cycle by a longitudinal jack, fast at one end with the mobile chassis and at the other end with the main chassis of the tamping machine.
A tamping machine designed according to this principle makes it possible to obtain a substantial increase in the operating performance with respect to tamping machines in which the working members are fixed with respect to the main chassis and therefore require, for their displacement from tie to tie, the acceleration and braking of the whole of the machine.
The continuously advancing tamping machines make it possible, in the phase of displacement of the working members from tie to tie, to be free of the conditions of wheel/rail adherence and, consequently, considerably to reduce the duration of this phase, since the efforts of acceleration or of deceleration of the working members are no longer to be transmitted in the form of driving torques acting on the wheels of the machine circulating on the rails. The systems of translation of the working units, characterizing this type of tamping machine, in fact enable much greater efforts of traction and of braking to be imparted to the working units.
However, a problem peculiar to this type of continuously advancing tamping machine resides in the compromise that must be found between the increase in output theoretically rendered possible by the principle set forth hereinabove of a chassis supporting the working members in relative movement with respect to the main chassis of the machine, and the comfort of the driving personnel who are in the cabs fast with the main chassis.
The accelerations and decelerations of the mobile chassis supporting the working members which appear at each work cycle lead, taking into account the appreciable weight of this mobile chassis, to longitudinal inertia reactions, with repercussions on the main chassis of the tamping machine and considerably shaking the drivers in the cabs at the beginning and end of the phase of displacement. The resulting discomfort is all the greater as it is desired to increase the output of the tamping machine further by increasing the speed of displacement of the mobile chassis. Another effect of these reactions of inertia is manifested in a loss of adherence of the main chassis rolling on the rails at the moment when they appear. In practice, this requires giving the main chassis as high as possible an adherent driving load and therefore leads to higher costs in driving the machine.